SPRING BIRD MIGRATION

By Leon Nawojchik, Manager, Ellanor C.
Lawrence Park

Spring is here. The days are lengthening and the temperatures are
warming. Now is an excellent time to see and study birds. Many of them
are at a peak of activity, have their bright-est colors and are very
vocal. You can visit natural park areas on your own or join a scheduled
nature center hike. Several sites have bird checklists showing what you
might expect to see. And during migration there is always the possibility
of a surprise sighting.

Why do birds migrate? Some don’t migrate at all. Many species like
backyard cardinals and chickadees are permanent residents, never
trav-elling far from home. Other species winter here from higher
latitudes or altitudes, enjoying our relatively warmer climate, and then
leave in the spring. Some neotropical migrants may travel hundreds or
thousands of miles to truly warm places.

The species’ choice of whether to migrate or not is a careful ecological
and evolutionary “calculation” worked out over thousands of years to
maximize success. Species that stay at home avoid the stresses and
dangers of long flights but have to endure reduced food supplies and
harsh weather. Active migrants may find abundant winter food by going
south but suffer from predation, storms, accidents, navigational errors
and physiological stress. Some long-distance fly-ers may lose up to
one-third of their body weight during migration. Generally those that
depend on food like caterpillars, fresh fruits or nectar, or on
conditions like warm weather, must leave.

How do they find their way? For thousands of years people have wondered
about and admired bird flight and migration. Over many years researchers
have uncovered several special avian abilities. Some species have been
shown to be sensitive to the earth’s magnetic field. Others can use the
nighttime star patterns to find their way, often back to the locality in
which they hatched.

Many small species travel at night, possibly to avoid hawks. Some larger
species like geese typically travel in daylight. Their V-shaped flocks
provide aerodynamic efficiency and leadership from experienced front
birds. These longer-lived geese make use of landmarks such as coastlines,
mountain ranges and river valleys that can be remembered from past trips.

Often birds may wait for changing winds to assist their flights. At
times they may be way off course. They may be subject to strong winds,
storm fronts, individual errors or quirks. During migra-tion season,
birders know that, if they are lucky, almost any species in the book may
show up.

Many individual birds do not make the trip. They may perish from
unexpected rain or ice storms. Some are caught by hawks or falcons.
Others may be blown out to sea. Man-made structures also take their toll.
Tall, lighted office buildings and lighthouses may be littered with the
bodies of dead songbirds that were confused and drawn in to a sudden
collision. Some locali-ties reduce lighting in tall structures during
peak migration hours.

The populations of many spring songbirds are declining. Human-caused
changes to the landscape such as development and forest fragmentation
take their toll here. Slash-and-burn agricultural prac-tices and the
clear-cutting of tropical forests are also reducing available wintering
grounds.

Recent climate changes affected by global warming are causing new
vegetation patterns, new weather patterns and changes in the dates of
emergence for leaves and insects. Timing is important. Typically an
increase in the length of the day is followed by the budding of young
leaves, then the emergence of insects and finally by the arrival of
birds. Some bird species will be able to adapt to changes in the timing
of these natural events. Others, especially those that are isolated or
limited in number, may not survive.

This spring many of us will make time to see and enjoy our colorful and
varied songsters. It is also an important moment to pause and reflect on
the complexities of migration and the con-servation needs of the future.